1. Field of the Invention
The present invention relates to a thin film transistor array substrate (hereinafter referred to as “TFT array substrate”) including a thin film transistor (hereinafter referred to as “TFT”) used as a switching element for active matrix type display device.
2. Description of the Related Art
As one of flat panel displays substituting for CRT, an electro-optical element for display including liquid crystal or electroluminescence has been extensively studied for application to products characterized by a low power consumption or small thickness. For example, a liquid crystal display device normally includes a liquid crystal panel having a liquid crystal layer interposed between a TFT array substrate and an opposite substrate having a color filter, a polarizing plate provided outside the liquid crystal panel and a backlight unit and a light source provided on the other outer side of the substrate. Light emitted by the light source is caused by the backlight unit to hit and pass through the polarizing plate and the liquid crystal panel to obtain a color image display. An electroluminescence display device includes an electroluminescent light-emitting layer and an opposite electrode formed on the aforementioned TFT array substrate. When electric current flows through the light-emitting layer, light can be displayed.
TFT's and pixel electrodes are aligned in a matrix pattern on the aforementioned TFT array substrate. The pixel electrodes each are connected to the drain electrode of the respective TFT. An interlayer insulating film is formed interposed between the drain electrode of TFT and the pixel electrode. The drain electrode of TFT and the pixel electrode are connected to each other through a contact hole formed in the interlayer insulating film. In general, the pixel electrode is made of a transparent electrically-conductive material such as indium oxide and zinc oxide. However, when the drain electrode is made of aluminum or aluminum alloy, an oxide layer is produced at the interface with the transparent electrically-conductive material, causing the rise of contact resistance. Therefore, as the material of drain electrode which comes in contact with the transparent electrically-conductive material there has been mostly used a high melting point metal having a reducing power such as chromium and titanium.
However, the recent trend is for more display devices including liquid crystal or electroluminescence to replace other display devices and find wider application. This trend is accompanied by the requirements for wider, finer and brighter screen. In order to maintain a good image quality even in a wide screen, it is necessary that the rise of the wiring resistance accompanying the increase of the screen size be inhibited. However, since the increase of the width of wiring causes the reduction of the light transmission area, i.e., percent opening and hence the reduction of brightness, it is necessary that a wiring material having a low resistivity such as aluminum or aluminum alloy be used.
Further, when the drain electrode is made of aluminum or aluminum alloy, there arises a problem that the contact resistance of the drain electrode with respect to the pixel electrode increases. In order to solve this problem, an approach has been proposed which includes forming an electrically-conductive film made of metal other than aluminum on the uppermost surface of the drain electrode (see, e.g., JP-A-4-253342 (lines 4-31, right column, page 3, FIG. 1)). To this end, however, it is necessary that a depositing or patterning step be added, leading to cost rise.
Further, an approach has been proposed which includes etching and removing the aluminum or aluminum alloy film on the uppermost layer of the drain electrode which has been exposed at the time of formation of a contact hole in the interlayer insulating film on the drain electrode (see, e.g., JP-A-9-244062 (line 12, right column, page 5-line 4, left column, page 7, FIGS. 5 and 6)). However, when such an isotropic etching method as wet etching is used, the horizontal side etching of the aluminum or aluminum alloy film proceeds, causing the side wall of the interlayer insulating film to be shaped such that it comes in no contact with and is separated from the drain electrode, that is, it is in the form of visor. Accordingly, the transparent electrically-conductive film which acts as a pixel electrode cannot be sufficiently covered, making it likely that poor conduction to the substrate metal film can occur.
On the other hand, it has been occasionally practiced to use not only light from backlight unit but also external light as a light source for the liquid crystal display device for the purpose of saving power consumption or enhance viewability in bright places. This process involves the formation of a transparent pixel electrode which transmits light as well as a reflective pixel electrode made of a material having a high light reflectance in the visible light range such as aluminum and silver on the upper part of TFT whereby external light incident on the liquid crystal panel is reflected and used as display light (see, e.g., JP-A-11-109417 (FIG. 1)). However, as can be seen from its configuration, the reflective pixel electrode is normally formed before or after the formation of the transparent electrode, requiring the addition of one step that adds to cost.
When the uppermost layer of the drain electrode is formed by aluminum or aluminum alloy so that it can act also as a reflective pixel electrode, the addition of such a step is not needed. However, the connection of the transmissive pixel electrode to the drain electrode raises a contact problem as mentioned above. Further, display devices including a reflective pixel electrode but free of transmissive pixel electrode, too, have a similar contact problem when packaged by covering the wiring terminal exposure area with an oxide-based electrically-conductive film such as indium oxide.